There exist two main systems for the sealing of ponds. The first makes use of natural materials, such as clay minerals or more specifically Na-bentonite: or soils rich in clay, salts or polymers (Prickett, 1968, FAO; Wilson, K. J., 1966, Am. Soc. of Civil Eng. Denver; Banin, A., 1976, Fac. Agri. Heb. Univ. Jerusalem; Garbotz, G., 1966. Con. Jour.). The second system is based on the use of artificial coverings of ponds, for example asphalt, plastic films: polyethylene, P.V.C. and synthetic rubber (Frobel, R. K., Clugg, C. B., 1976, Jour Irri. Drain Div; Hickey, M. E., 1969, U.S.D. Int. Bur. Rec. R.R.19; Kumar, J., Jedlick, A. 1973, Agri. Res.Ser. USDA).
The first system suffers from seepage of water, reaching several percentages of water loss per volume. In cases in which sewage water is restored there exists the danger that such water will reach the aquifer and contaminate it. The artificial materials used for sealing are more reliable but much more expensive than the natural ones.
In many studies (McNeal, B. L. and Coleman, M. T., 1966, Soil Sci. Soc. Am. J. 30, 308; Frankel H. et al., 1978, Soil Am. J. 42,32; Porather, R. I. et al., 1978, Soil Sci. Am. J. 45,273; Pupisky H. and I Shainberg, Soil Sci Soc. of Am. J. 43,429), the combination of soil material and clay with the addition of Na salts, was found to minimize the hydraulic conductivity of porous media. However, the exact percentages of clay and sand were not specified. Absolute saturation of the clay by Na was never attained, nor obviously the absolute sealing of the soil, and its application for water reservoirs.